i965: Track when a bo is shared with an external client

i965: Track when a bo is shared with an external client

If the buffer is being shared with an external client, our own state
tracking may be stale and in some cases we may wish to double check with
the kernel/hw state. At the moment, this is synonymous with not being
reusable, but the semantics between reusable and external are quite
different and we will have more examples of non-reusable buffers in the
near future.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>